The invention relates generally to electric motors and, more particularly, to start procedures for use with electric motors.
Many modern devices make use of direct current (DC) brushless motors to provide rotational movement within the device. For example, hard disk drives commonly use DC brushless motors as spindle motors to rotate a disk carrying hub about an axis of rotation. A DC brushless motor typically includes a movable rotor portion having permanent magnet structures disposed about a circumference thereof and a stationary stator portion having a plurality of coils wound thereon in fixed relation to one another. By properly driving the coils on the stator, the rotor portion is set in motion to revolve about an axis of rotation in a desired manner.
In a DC brushless motor, the specific motor coils that need to be energized at any particular time to support rotation of the rotor typically depend upon the present rotational position of the rotor with respect to the stator. Therefore, to efficiently power a DC brushless motor, it is usually necessary to track the position of the rotor with respect to the stator so that proper commutation of the motor coils can be achieved. Traditionally, back electromotive force (EMF) generated in the coils during rotation of the rotor has been used to provide rotor position feedback for use in determining the proper commutation points for the motor. However, back EMF is unreliable at low rotor rotation velocities, such as those that exist during motor spin up operations. Consequently, rotor position feedback has not traditionally been used during the early stages of rotor acceleration, thus making accurate commutation difficult and inefficient during spin up operations.
Therefore, there is a need for a method and apparatus for efficiently and accurately spinning up a DC brushless motor.
The present invention relates to a system and procedure for use in spinning up a DC brushless motor. During the early stages of the rotor acceleration process (i.e., the spin up period), the position of the rotor is repeatedly sampled for use in identifying the ideal commutation points of the motor. The sampling interval that is used to sample the rotor position is decreased as the rotor accelerates so that the position update rate roughly tracks the increasing angular velocity of the rotor. In a preferred embodiment, the sampling interval is changed based upon a changing commutation state of the motor. In alternative embodiments, the sampling interval is changed based upon the changing position or speed of the rotor or based upon elapsed time.
Because the rotor is moving more slowly at the beginning of the spin up process, the rotor position update rate that is required for accurate commutation during this time is lower. As the rotor begins to move faster, the position update rate is increased (i.e., the sampling interval is decreased) so that more accurate commutation can be achieved at the higher speeds. Eventually, the commutation function is preferably switched over to a conventional back EMF based position feedback approach. Because position sampling is less frequent at the start of the spin-up procedure, the relative amount of time that is dedicated to the application of drive current to the motor windings is larger at the beginning of the process. Maximizing the percentage of time that the drive current is applied while the rotor is stationary or moving slowly increases the motor""s ability to overcome loading or friction problems (e.g., stiction events in disk drives) during spin up. Thus, a highly reliable and efficient spin up of the motor can be achieved.